1. Field of the Invention
The present invention relates to an oscillator. More particularly, the present invention relates to a low temperature coefficient oscillator.
2. Description of Related Art
FIG. 1 is a schematic diagram illustrating a conventional silicon oscillator. Referring to FIG. 1, the silicon oscillator 100 charges a capacitor through a current I. When the capacitor is charged to a predetermined voltage V, a switch is triggered and is turned on, so that capacitor is discharged to the ground GND. Then, the switch is turned off, and the capacitor is recharged. By such means, the capacitor is repeatedly charged/discharged, so that the silicon oscillator 100 can generate a fixed frequency f (f˜|I/(C*V)).
If the frequency f not influenced by temperature variation is required to be generated, the current source circuit 110, the voltage source circuit 120 and the capacitance of the capacitor have to have very small temperature coefficients. FIG. 2A is a schematic diagram illustrating the current source circuit 110 of FIG. 1. FIG. 2B is a schematic diagram illustrating the voltage source circuit 120 of FIG. 1. Referring to FIG. 2A and FIG. 2B, the current source circuit 110 can generate a low temperature coefficient current source I based on the ratios among resistors R11 and R21, the positive temperature coefficient characteristic thereof and the negative temperature coefficient characteristic of the transistors Q11 and Q12. Moreover, the voltage source circuit 120 can generate a low temperature coefficient voltage source V based on the ratios among resistors R11, R31 and R41, the positive temperature coefficient characteristic thereof and the negative temperature coefficient characteristic of the transistors Q11 and Q12. Though, all of the aforementioned circuits can be integrated within a signal circuit, the voltage source and the current source with low temperature coefficient have to be applied, so that a circuit area of the oscillator 100 is relatively large during a chip fabrication.
Moreover, a U.S. Patent issued as U.S. Pat. No. 7,034,627 discloses an oscillator circuit (not shown) in which a low temperature coefficient constant resistance circuit is applied to achieve a low temperature coefficient frequency output. Since currents generated by a current mirror circuit in the oscillator circuit have the same temperature coefficient, the voltage and charge current used for generating the output frequency of the oscillator circuit also have the same temperature coefficient, and since the resistor generally has a relatively high temperature coefficient during a semiconductor fabrication process, if the resistor is integrated to the low temperature coefficient frequency oscillator circuit during the semiconductor fabrication process, the output frequency generated by the oscillator circuit can be influenced by the temperature, which may cause a wrong operation of the circuit.